The present invention relates generally to water management within a fuel cell, and more particularly to ways to remove water from moisture-rich reactant flowpaths.
In many fuel cell systems, hydrogen or a hydrogen-rich gas is supplied through a flowpath to the anode side of a fuel cell while oxygen (such as in the form of atmospheric oxygen) is supplied through a separate flowpath to the cathode side of the fuel cell. An appropriate catalyst (for example, platinum) ionizes the hydrogen into a proton and electron on the anode side such that upon subsequent combination of the proton with oxygen and the electrons at the cathode side, electric current is produced with high temperature water vapor as a reaction byproduct.
In one form of fuel cell, called the proton exchange membrane fuel cell (PEMFC), an electrolyte in the form of a ionomer membrane is assembled between electrodes known as an anode and cathode. This layered structure is commonly referred to as a membrane electrode assembly (MEA), and is further layered between bipolar plates to allow communication with the respective anode and cathode reactants. The bipolar plates separating each MEA include channels formed in opposite surfaces. These channels act as conduit to convey hydrogen and oxygen reactant streams to the respective anode and cathode of the MEA. In addition to providing flowfield channels to act as reactant flowpaths, the bipolar plates can be made electrically conductive to act as current collectors for the generated electricity in the regions of the plates that are adjacent electrochemically active area of the MEA. Layers of porous support material and catalyst are situated between the channels of the plates and each side of the membrane to facilitate the necessary electrochemical reactions.
In a PEMFC stack, the water produced in the oxygen reduction reaction at the cathode is removed through the flowfield channels formed in the bipolar plates. The highest concentration of water exists at or near the outlet regions of the plates, due in part to low gas velocities that reduce the purging quality of the gas. Under such conditions, the likelihood of liquid water stagnating and accumulating in the exit region of the plate is greater, which is undesirable in that by plugging up the flow channels with water droplets, it adversely impacts stack voltage stability. In addition, stack durability is impacted, as flow blockage entailed by such droplets can cause localized hydrogen starvation and related carbon corrosion. Furthermore, the accumulation of water that is exposed to sub-freezing conditions for prolonged periods leads to ice buildup within the flowfield channels, thereby inhibiting operation. Present methods of avoiding this condition include maintaining a high gas velocity to purge the excess liquid from the stack flowpath, and operating the stack under extremely dry conditions. The first is disadvantageous in that it necessitates additional power consumption to operate a compressor or related pumping device, while the second is disadvantageous in that it could upset the delicate humidity balance required in the ionomer. It is therefore desirable that a PEMFC stack be configured to reduce or eliminate the buildup of excess water in the flowfield channels of the bipolar plates without the disadvantages mentioned above.